This invention relates to a method for the production of a sintered article of silicon nitride.
The sintered ceramic articles formed preponderantly of silicon nitride possess heat resisting property enough to withstand elevated temperatures up to 1900.degree. C., exhibit a low thermal expansion coefficient, and excel in resistance to thermal shock. Thus, the feasibility of these articles in applications to structural parts such as gas turbine blades and nozzles which require high strength at elevated temperatures is being studied.
Shaped articles formed solely of silicon nitride powder have a poor capacity for sintering. It has been customary, therefore, to produce sintered articles of silicon nitride powder by blending the silicon nitride powder with a sintering aid such as the oxide of a rare earth element like yttrium oxide (Y.sub.2 O.sub.3), aluminum oxide (Al.sub.2 O.sub.3), or aluminum nitride (AlN), molding the resultant mixture in a given shape, and sintering the shaped mixture by the hot press method, the normal sintering method, or the gas pressure sintering method.
As means of forming such shaped articles of silicon nitride, numerous methods have been known such as the pressure molding method applying of pressure to powder, injection molding method using blended materials and the slip casting method using slurry blends. Particularly, the molding method involving slurry blends is known for its ability to permit such slurry blend to be easily and inexpensively molded in a complex shape.
When silicon nitride powder and an aluminum nitride sintering aid are mixed and graded in water to produce a slurry, the aluminum nitride reacts with water and undergoes decomposition, converting itself into Al.sub.2 O.sub.3 with evolution of ammonia.
When this decomposition occurs, the amount of aluminum nitride present in the mixture becomes smaller than the amount initially incorporated therein. The sintered article finally obtained, therefore, fails to acquire properties as expected and suffers from dispersion of properties. Moreover, the offensive odor which emanates from the shaped article because of the evolution of ammonia seriously impairs the work environment. Consequently, when the blended material of silicon nitride containing aluminum nitride is subjected to slip casting, the blend must be converted into a slurry using an organic solvent. This unavoidable of the organic solvent results in additional cost.
Aluminum nitride functions effectively as a sintering aid particularly when the sintered article of silicon nitride is produced by sintering in a furnace. Thus, the desirability of developing a method which effects slip casting inexpensively with use of aluminum nitride as the sintering aid has been increasingly recognized.